Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2002-12-03
2004-08-10
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
Reexamination Certificate
active
06774842
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for generating entries for a database, which database is destined for supporting a positioning of a mobile terminal, in particular a hybrid positioning of a mobile terminal. The invention relates equally to a unit realizing such a method and to a system realizing such a method.
BACKGROUND OF THE INVENTION
In a hybrid positioning of a mobile terminal, data from a main positioning system, e.g. a satellite based positioning system, are combined with data from a cellular network in order to determine the position of the mobile terminal.
A well known satellite based positioning system is GPS (Global Positioning System). In GPS, code modulated signals are transmitted by several satellites that orbit the earth and received by GPS receivers of which the current position is to be determined. Each of the satellites transmits two microwave carrier signals. One of these carrier signals L
1
is employed for carrying a navigation message and code signals of a standard positioning service (SPS). The L
1
carrier signal is modulated by each satellite with a different C/A (Coarse Acquisition) Code known at the receivers. Thus, different channels are obtained for the transmission by the different satellites. The C/A code, which is spreading the spectrum over a 1 MHz bandwidth, is repeated every 1023 chips, the epoch of the code being 1 ms. The carrier frequency of the L
1
signal is further modulated with the navigation information at a bit rate of 50 bit/s. The navigation information comprises in particular ephemeris data. Ephemeris parameters describe short sections of the orbit of the respective satellite. Based on these ephemeris parameters, an algorithm can estimate the position and the velocity of the satellite for any time of about 2-4 hours during which the satellite is in the respective described section. Ephemeris data also comprise clock correction parameters which indicate the current deviation of the satellite clock versus a general GPS time.
Further, a time-of-week TOW count is reported every six seconds as another part of the navigation message.
A GPS receiver of which the position is to be determined receives the signals transmitted by the currently available satellites, and a tracking unit of the receiver detects and tracks the channels used by different satellites based on the different comprised C/A codes. The receiver first determines the time of transmission of the ranging code transmitted by each satellite. Usually, the estimated time of transmission is composed of two components. A first component is the TOW count extracted from the decoded navigation message in the signals from the satellite, which has a precision of six seconds. A second component is based on counting the epochs and chips from the time at which the bits indicating the TOW are received in the tracking unit of the receiver. The epoch and chip count provides the receiver with the milliseconds and sub-milliseconds of the time of transmission of specific received bits. A detected epoch edge also indicates the code phase of a received signal.
Based on the time of transmission and the measured time of arrival TOA of the ranging code at the receiver, the time of flight TOF required by the ranging code to propagate from the satellite to the receiver is determined. By multiplying this TOF with the speed of light, it is converted to the distance between the receiver and the respective satellite. The computed distance between a specific satellite and a receiver is called pseudo-range, because the general GPS time is not accurately known in the receiver. The computed distances and the estimated positions of the satellites then permit a calculation of the current position of the receiver, since the receiver is located at an intersection of the pseudo-ranges from a set of satellites.
In weak signal conditions, however, GPS positioning cannot be carried out in a standalone fashion in the receiver. Assistance of some kind is needed to recover the positioning capability. If the GPS receiver is part of or connected to a mobile terminal operating in a cellular communication network, the simplest form of GPS assistance is to deliver navigation data over the cellular network to the receiver. Usually, missing navigation data is the key element why positioning cannot be maintained or initiated in weak signal condition for a long period.
A more sophisticated form of assisting the receiver is a delivery of the exact GPS time to the receiver. Exact time is needed e.g. to improve the sensitivity of the receiver. However, along the exact time also a reference location of some quality is basically mandatory. The reference location, i.e. a known position near to the expected location of the receiver, is needed for calculating geometrical distances between the satellites and the receiver. The calculated distances are then used for predicting navigation data bit edges and C/A-code phases, in order to improve the sensitivity of the receiver and to speed up the signal acquisition.
The availability of a reference location is thus a key factor for some time recovery and sensitivity improvement methods in assisted GPS. If the GPS receiver is part of or connected to a mobile terminal operating in a cellular communication network, the coordinates of the cell in which the mobile terminal is known to be located could be used as reference location. However, while a cellular communication network usually provides a mobile terminal with an identification of the cell in which it is currently located, this identification does not contain information about the geographical location of the cell. The geographical location information of the cells in a cellular network is usually controlled by the network operators. Thus, in order to obtain a reference location from the network, it is necessary to poll the location from the network, which might be time consuming. Especially in the case of emergency calls a delay in signal acquisition may be critical. Moreover, the network operators offer the position information usually as a chargeable service to their subscribers, unless it is required for an emergency call.
If an available reference location is too far from the receiver, it decreases the possibilities to assist GPS hardware in acquisition and in tracking and prohibits the use of some time recovery methods in the case that the time assistance is not exact. Thus, it is also useful to know the reliability of a provided reference position, i.e. the maximum distance from the current position of the receiver to the provided reference location. If the accuracy of the reference location is known to be good enough for some application, the reference position can even be used as such, and GPS is not required at all.
In conventional positioning systems, the accuracy of the reference location is either provided by a cellular communication network or not available at all.
Similar problems may also arise with other positioning systems than GPS.
In order to avoid the necessity of polling the cellular network each time a reference location is required, European patent application EP 1 237 009 A2 introduced the idea of a cell location database. The proposed database is used to store the geographical position of cells. In the Global System for Mobile Communication (GSM), each cell has a unique Cell Global Identity (CGI) identification. When a CGI has once been associated with a geographical position, this position can be stored and used as reference location also later on, whenever the receiver is in the coverage area of the cell with this particular CGI. The database can be stored in the non-volatile memory of a mobile terminal or be downloaded from a network independently from the operator of the cellular network, e.g. using the Wireless Application Protocol (WAP). The cited patent application, which is incorporated by reference herein, also provides a detailed description on how the geographical information in the database can be used as reference location in the positioning of a mobile terminal. It does not
Nokia Corporation
Phan Dao
Ware Fressola Van Der Sluys & Adolphson LLP
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